1. Field of the Invention
The present invention relates to a stereoscopic endoscope by which a part to be inspected such as the inside of a body cavity is stereoscopically observed by means of a pair of objective optical systems disposed at the front end of its inserting portion; and, more specifically, to a structure of the front end part of the inserting portion thereof.
2. Description of the Prior Art
Recently, surgical techniques in which endoscopes are used for observing parts to be inspected within body cavities and performing operations on diseased parts within the body cavities have been prevailing. For making diagnoses more accurately and alleviating the suffering of patients, the endoscopes have been technically improved. In particular, for performing a diagnosis more accurately within a body cavity, it is useful to obtain depth information within the body cavity. Stereoscopic endoscopes have been known to respond to such a demand.
In a stereoscopic endoscope, for the sake of workability, a pair of objective optical systems corresponding to the right and left eyes are assembled therein with their optical axes being parallel to each other. When such parallelism is secured in their optical axes, it is necessary that the base length formed by the optical axes of the pair of objective optical systems be remarkably shortened in view of easiness in viewing.
In practice, however, it has been difficult to extremely shorten the base length due to mechanical restrictions. As a consequence, easily viewable three-dimensional images have been hard to obtain.
In order to overcome such a problem, the assignee has already proposed a stereoscopic endoscope in which a convex front end lens is disposed on the object side of a pair of objective optical systems, so that an easily viewable three-dimensional image can be obtained even when the base length formed by the optical axes of the pair of optical systems is not set short (Japanese Unexamined Patent Publication No. 6-331939).
In such a stereoscopic endoscope, however, the front end lens is disposed at the front end face of the front end part of the endoscope and thereby occupies the major space of the front end face.
In general, the front end face of the front end part of an endoscope is formed with an air/water discharge hole, through which air or washing water is discharged. A space for arranging the air/water discharge hole is necessary in the front end face of the front end part in the above-mentioned stereoscopic endoscope having the front end lens as well.
When the head of a screw for securing the front end part to a front end part main body is to be positioned at the front end face of the front end part, a space therefor is also needed.
Further, an illumination optical system for illuminating an object to be viewed is disposed at the front end part of such an endoscope. As shown in FIG. 9, when a front end lens 111 is disposed on the object side of an illumination optical system 113, light beams 120, of the light from the illumination optical system 113, reflected by the object-side face of the front end lens 111 enter the inside of an objective optical system 130 from its front end face, thereby generating noise light with respect to the original subject image light and deteriorating the image quality. Therefore, it is desirable that the front end part of such an illumination optical system 113 be positioned on the front end face of the front end part of the endoscope.